1. Field of the Invention
The present invention relates to an apparatus and method for polishing substrates. More particularly, the invention relates to a platen/polishing pad assembly having surface to improve polishing uniformity of substrates.
2. Background of the Related Art
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited and removed from a substrate during the fabrication process. Often it is necessary to polish a surface of a substrate to remove high topography, surface defects, scratches or embedded particles. One polishing process is known as chemical mechanical polishing (CMP) and is used to improve the quality and reliability of the electronic devices formed on the substrate.
In general, the polishing process involves holding a substrate against a polishing pad under controlled pressure, temperature and rotational velocity of the pad in the presence of the slurry or other fluid medium. Typically, the polishing process involves introducing a chemical slurry during the polishing process to facilitate higher removal rates and selectivity between films on the substrate surface. One polishing system that is used to perform CMP is the MIRRA.RTM. System available from Applied Materials, Inc.
An important goal of CMP is achieving uniform planarity of the substrate surface. Uniform planarity includes the uniform removal of material from the surface of substrates as well as removing non-uniform layers which have been deposited on the substrate. Successful CMP also requires process repeatability from one substrate to the next. Thus, uniformity must be achieved not only for a single substrate, but also for a series of substrates processed in a batch.
Substrate planarity is determined, to a large extent, by the construction of the CMP apparatus and the composition and construction of the consumables such as the slurry and the pads, all of which contribute to the polishing rate. One factor which contributes to non-uniform polishing is the non-homogeneous replenishment and distribution of slurry at the interface of the substrate and the polishing pad. The slurry is primarily used to enhance the material removal rate of selected materials from the substrate surface. As a fixed volume of slurry in contact with the substrate reacts with the selected materials on the substrate surface, the slurry constituents are consumed.
Non-uniform slurry distribution on the pad results because the inertia of the slurry during the rotation of the pad causes the slurry to flow off of the pad during operation. As a result of the non-uniform slurry distribution on the pad, the substrates being processed experience poor polishing uniformity. Usually, the resulting polishing is "center-slow," meaning that the removal rate of material at the center portion of the substrate is lower than at the outer potion of the substrate. Attempting to compensate for the center-slow polishing effect by increasing the pressure applied to the center portion of the substrate can compromise the planarity of the substrate because the proper properties of pressure are difficult to achieve. In addition, the increased loading pressure of the substrate on the pad also acts to force the slurry out from between the pad and substrate leaving areas on the pad starved of slurry. As a result, the polishing is non-uniform over the surface of the substrate.
One solution to remedy the problem of poor slurry distribution has been to provide grooves in the pad. One grooved pad is the IC 1000 available from Rodel, Inc., of Newark, Del. FIG. 1 shows an X-Y configuration of the grooves formed in the upper polishing surface of the pad. A plurality of the grooves extend parallel to one another in a first direction (X) and a plurality of grooves extend parallel to one another in a second direction (Y), which is orthogonal to the first direction. The result is an X-Y pattern of grooves intersecting one another at right angles. The grooves are believed to control the distribution of the slurry during operation by retaining a portion of the slurry in the grooves. However, while such pad designs accommodate more slurry volume than flat or planar pads, the pads have proved ineffective in achieving uniformity in slurry distribution because the inertia of the slurry causes the slurry to flow radially outward and off of the pad during rotation of the pad.
In an attempt to ensure the uniform distribution of fresh slurry to all areas of the substrate, conventional methods continually supply large volumes of slurry to the pad during a polishing cycle. As a result, slurry is the primary consumable in chemical mechanical polishing and a significant source of the cost of operation. In order to minimize the cost of operation, the volume of slurry used in a processing cycle should be minimized. However, as noted above, conventional pads are not capable of efficiently retaining the slurry between the pad and the substrate. As a result, the volume of consumed slurry is substantially higher than is desirable.
Another problem with the presence of grooves on the polishing surface of a pad is the detrimental effect on the polishing characteristics of the pad. In particular, the grooves decrease the total area available for polishing the substrate, thereby decreasing the removal rate of material from the substrate. Further, the stiffness of the pad can be affected by the grooves. A preferred pad construction allows for a proper balance between rigidity (or stiffness) and compliance (or flexibility) of the polishing pad. In general, stiffness is needed to ensure within-die uniformity which refers to the ability of the CMP apparatus to remove features across the diameter of the substrate regardless of substrate shape and/or topography across its surface. The provision of grooves on the polishing surface can decrease the stiffness of the pad to an unacceptably low level, resulting in poor within-die uniformity.
Therefore, there is a need for a polishing pad capable of controlling slurry distribution over the pad surface and providing uniform and planar polishing.